Self-aligned contacts (SAC) allow the design of a semiconductor device to have a distance between the gate and the contact to the substrate, to be at most one-half the minimum gate width. Typically, SAC uses a nitride layer on the gate stack, together with spacers that include nitride, to prevent a misaligned contact from contacting the gate itself. If the nitride were not present, then the etch used to form the hole which will become the contact would pass through the dielectric layer all the way to the gate, when misaligned. When present, the nitride layer and spacers acts as an etch stop, preventing misalignment form forming a hole all the way to the gate, and therefore allowing design of the device to have a much smaller average distance between the contact and the gate.
Chemical vapor deposition (CVD) of silicon nitride at low temperatures is conventionally carried out in a vertical furnace, using gases containing a silane, for example bis-(t-butylamino) silane (BTBAS) and ammonia (NH3). Typically, each zone of the furnace is maintained at a constant, relatively low temperature, during the deposition. A cold feed of NH3 and BTBAS is introduced at the bottom of the furnace through a distribution tray. The low temperature deposition reduces the overall thermal budget in forming devices, preventing damage to the device, and is especially important as device dimensions are reduced. For example, U.S. Pat. No. 6,803,321 to Blosse et al. issued 12 Oct. 2004 describes low temperature CVD of silicon nitride films for spacers.
This current approach produces non-uniform silicon nitride films across the wafer, and from wafer to wafer within a batch of wafers processed simultaneously in a vertical furnace. It is believed that due to the colder temperature of the process gas at a lower or bottom zone of the furnace during silicon nitride deposition, less hydrogen (H) is incorporation into the silicon nitride film at the bottom zone. This lower hydrogen content results in a lower RI (refractive index), and a greater etch rate when using a fluorine containing gas (such as CHF3/O2 chemistry, typically used for spacer overetch due the relatively high selectivity of this etching chemistry to silicon oxide (SiO2)). Table 1 shows the properties of silicon nitride films deposited on a batch of wafers, by location in the furnace during deposition. Shrinkage is the percent shrinkage of the film thickness after heating at 1000° C.
TABLE 1Location inSpacer etch, overetchShrinkagefurnace(angstroms)(%)RIH contentTop14811.031.92952.53E+21Center16010.961.91782.04E+21Bottom18711.811.89091.59E+21Standard12.174.191.0322.8deviation(%)
To compensate for the lower etch rate of wafers processed at the top and/or the center part of the furnace during silicon nitride deposition, a longer overetch is used, which in turn reduces the amount of silicon nitride over the gate stacks, and in spacers surrounding the gate stacks. This loss of silicon nitride during etching of silicon nitride produced by conventional low temperature CVD requires that the distance between the gate stack and the contact in self-aligned contacts, be maintained to minimize the contact-to-gate leakage.